I just went though this.
L2501-
L3901 is the same chassis/frame, as I'm sure you discovered.
I ended up with a
L3301. It was the "most tractor" that fit into my budget. I dont recall what the jump in price for the 3901 was, but it was enough to bump it out of my spend range after I added in options and implements.
While I did think similar things about the
L2501, the pto power was limiting right away. Im sure you might be able to change out gearing, but I would guess you'd immediately loose your warranty. So you'd either be out that, or have a tractor with less pto than what you maybe wanted for the time till you swapped things out.
My thoughts were the differences between the 33 and 39 are probably computer controlled, as the bore/stroke and displacement are identical. Someone might come out with an "engine tune" that would bump the power up at some point. But the power of the 33 does everything I need it to do right now and it fit my budget.
The
L4701 jumps to a bigger chassis/frame.
If you have an open budget, i don't see a reason to not go as big as you can.